Species differences in inhibitory effect of Li+ on high K+-induced contraction in the ileal longitudinal smooth muscle

The effects of Li+ substitution for external Na+ in the medium on a 60 mN KCl (60K+)-induced contraction were compared in the ileal longitudinal smooth muscles isolated from monkey, rabbit, guinea pig and rat. Li+ substitution inhibited the 60K+-induced contraction in the ileum of all the animal species. Greater inhibitory effect was observed with the increase in external Li+ concentration and the increase in incubation period with Li+. Intracellular Li+ contents of these muscles also increased with the increase in external Li+ concentration and the incubation period. Thus, a good correlation was obtained between Li+-induced inhibition of 60K+-induced contraction and the increase in Li+ content. However, close examination revealed that the contraction in monkey and rat ileum was inhibited by much lower Li+ accumulation than that in guinea pig and rabbit ileum, suggesting the existence of species difference in the sensitivity of the ileal smooth muscle to the inhibitory effect of Li+ substitution, and a mechanism for the species differences in the inhibitory action of Li+-substitution was discussed.